A pilaster, pier, or body of masonry projecting beyond the main face of the wall and intended to strengthen the wall at particular points and also to counterbalance the thrust of a roof or its vaulting. The term "counterfort" is used when the projection is on the inside. A flying buttress is an arch, resting at one end on a detached pier and it carries the thrust of the nave vault over the aisles or cloister.